The present invention relates in general to vapor generating equipment, and more particularly to a panel of vapor generating and superheating tubes in or for use in such equipment.
Significant effort has been directed in recent years toward the development of a solar receiver suitable for power generation. One of the more promising concepts is that of a solar power tower with a central receiver located on a tower. Sunshine falling on an array of computer guided tracking mirrors known as heliostats is reflected toward the receiver and the incident energy is absorbed by a heat transport fluid flowing in the receiver. Thermal energy in the fluid may be used to run a turbine generator or to provide heat for industrial use.
It is important that the solar receiver deliver the working fluid, usually steam, at a constant temperature and pressure in order for it to be acceptable for turbine operation. Water/steam receivers usually consist of three basic parts connected in series: a preheater comprised of a group of economizer tubes; an evaporator comprised of a group of steam generating tubes; and a superheater. Steam generators are discussed in Steam, Its Generation And Use, 39th Ed., The Babcock & Wilcox Company, 1978.
In order to maintain a constant temperature of the steam leaving the superheater, the amount of heat delivered to the superheater and to the evaporator should be properly proportioned. This proportion should remain nearly constant for all operating conditions.
Although steam generation techniques are generally well known, solar receivers operate in an environment which is different from that of fossil or nuclear steam generators. A solar receiver is exposed to daily cycling from 0 to peak power with a multitude of fast variations of heat distribution due to cloud transients. In known solar receiver designs, the panels of tubes are arranged at fixed locations on the receiver, the locations being based on clear day insolation conditions. When, during a cloud transient, the heliostat field is partially shaded, the heat distribution to the tube panels becomes unbalanced resulting in loss of control of the steam temperatures from the superheater tube panels. Because of the decreased power input to the receiver from insolation, steam generation is reduced, but the heat flux from insolation onto some receiver areas may remain high. If high heat flux on some superheater tubes coincides with high steam temperature due to reduced steam flow therein, the superheater tube metals may become overheated and overstressed resulting eventually in tube failure.
The prior art has sought to alleviate the problems discussed above, at the expense of high pressure losses and less efficient steam generation, by such means as requiring extremely high fluid flows through all of the tubes to compensate for random high heat flux in some of them, increasing the size of the solar receiver, or defocusing most of the bright heliostats from the receiver resulting in little power generation during the transient conditions.
As shown in my U.S. Pat. No. 4,245,618, it is known to provide a panel of evaporator screen tubes in front of and spaced from a panel of superheater tubes wherein the superheater tubes are provided with webs, or strips of metal sometimes known as membranes, between and along the lengths of the tubes for additional heat absorption. The membranes are also required to prevent entry of the insolation to the rear of the tubes where damage may result to the receiver as well as loss of energy. However, the use of membranes results in less efficient duplication of heating surface, the cost of membraning the superheater tubes is quite high, and additional stresses may occur in the membranes if there are temperature differentials between adjacent superheater tubes. In addition, a conventional buckstay arrangement cannot be provided to maintain appropriate innertube spacing and inhibit vibration of the screen tubes since such an arrangement would be undesirably exposed to high insolation. Instead, support bars may be welded on the back of individual screen tubes respectively which bars extend through slots in webs of the corresponding superheating tube panels for attachment to complex and expensive individual vibration support structures in back of the superheating tube panels which structures are in turn attached to the respective webs.
The indicent heat rays on the absorber surface of a central solar receiver with the heliostat field arranged northwardly of the tower are predominately parallel and only a small fraction thereof deviate from the predominate direction. When screen tubes are located in front of a superheater panel, they shade the superheater panel surface behind the screen tubes so that a superheater tube located in a shaded area will absorb a lesser amount of heat while a superheater tube located in an unshaded area of the panel will absorb a greater amount of heat. Since the steam flow per tube is about the same for adjacent superheater tubes, the temperature of the steam leaving the tubes absorbing more heat is much higher than the temperature of the steam leaving the shaded superheater tubes. This would require a higher alloy material, which is more expensive, to be provided for the superheater than would be required if the heat distribution to all tubes in a panel were uniform. In addition, temperature difference between adjacent tubes of the superheater panel may be great enough to cause excessive metal stresses which may lead to tube failures.
An object of the present invention is to avoid the difficulties mentioned above while eliminating the economically unattractive alternatives of the prior art in order to provide a significant advantage for reliable receiver operation.
It is another object of the present invention to arrange the components of a tube panel for a solar receiver so as to assure a proper distribution of incident heat flux among the vapor generating and superheating tubes while eliminating complex and expensive structure such as membranes for the superheating tubes and/or the aforementioned complex and expensive vibration support structure.
It is still another object of the present invention to provide proper heat distribution during severe cloud transients to maintain exit vapor temperatures from the superheating tubes of a solar receiver within allowable limits.
It is yet another object of the present invention to arrange the components of a solar receiver so as to assure good heat transfer while preventing overheating of the tubes during the various conditions to which a receiver is normally subjected.
It is another object of the present invention to arrange the tubes of a receiver panel so as to assure good heat transfer while preventing overheating of the tubes during the various conditions to which a receiver is normally subjected.
It is another object of the present invention to provide an inexpensive yet effective tube panel for use on the furnace walls of fossil-fired vapor generators.
It is another object of the invention to provide a simple panel construction which permits large relative longitudinal differential movement of superheater tubes relative to vapor generating tubes without excessive vibration and bowing of tubes due to temperature differentials, wind, or gas flow loading.
It is another object of the present invention to minimize the loss of infrared radiation and convection losses from a vapor generating and superheating tube panel for greater thermal efficiency of a solar receiver in which the panel is installed.
It is yet another object of the present invention to provide a tube panel for a solar receiver as well as for other types of vapor generators heated predominately by radiation which is simple in design, rugged in construction, economical to manufacture, and economical to operate.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.